95% of the problems sit.... Have you tried to delete the prv file in the folders where that happens Monsoon? They contain the previews and will be rebuilt when they are not there. Sometimes a good way to reset collection previews. AND, i guess you are sure that the disappearing materials were not 3.1 ones that 5 and higher can't read and therefore "strips" out. Right?

One day your ship comes in - but you're at the airport.